What is HDR?

HDR defined

HDR stands for High Dynamic Range. HDR photography is a specific way of processing pictures that has become quite popular lately thanks to photography going digital and the emergence of several new software tools. You have probably seen HDR photos but not realized it. Some look “different” as one of the side effects of this special treatment results in a somewhat surreal looking image, that has an almost “painterly” feel to it. However, this “look” is often created as an artistic effort and sometime out of ignorance as it is quite possible to get “natural” looking images from this process.

In this article we take a look at what HDR is and why we’d want to do it.

Let’s start with a definition. In photography we can define High Dynamic Range as follows:

The difference in luminosity between the darkest areas and the lightest areas in a scene.

We can therefore immediately conclude that HDR can apply to B&W as well as color images, as it is all about light and dark.

Let’s put this into some context. Take a look at the following table, useful for photographers. In this table, we introduce the term “EV” or Exposure Value, often expressed in “stops”. One stop is a doubling or halving of the amount of light. We already know this term and use it every time we set our exposure on our camera. A shutter speed of 1/30 is twice as long (or double) as that of 1/60 and an aperture of f5.6 is half the size of f/4.

Scene

Dynamic Range

Stops (EV)

Typical Outdoor (sunlit scene)

100,000:1 or more

~17 EV

Human Eye

10,000:1

~14 EV

Negative Film

Up to 2000:1

~10-11 EV

Digital Camera

Up to 400:1

~6-9 EV

Computer Monitor

500:1 to 1000:1

~9-10 EV

Quality Gloss Photo Print

100:1 to 250:1

~7-8 EV

Quality Matte Photo Print

50:1

~5.6 EV

Over at DPReview.com, the internet source for extensive camera reviews, we can find the following graph that illustrates the dynamic range of some popular cameras. According to DPreview’s tests, the Canon 5D Mark2 scores a DR of 8.4EV, The Sony Alpha A900 9.4EV, The Nikon D700 7.8EV, the Canon 5D, 8.2EV and the Canon 1Ds Mark III, 8.6EV.

What does this all mean?

Now that we’ve defined what Dynamic Range is and what the dynamic range is of our cameras, let’s now look at the practical side and answer the question “What does this all mean for my pictures?”

Here is a typical scene, I am sure we all have taken pictures in similar circumstances: A bright sky and shadowed areas.

The accompanying histogram for this shot looks like this:

This histogram clearly shows pixels stacked up against both the left and right margins. In a histogram, the area between the Left margin (blacks) and the Right margin (whites) represents the dynamic range of the camera’s sensor. The graph itself represents the pixels that capture the scene we’re shooting. The taller the “stack” the more pixels.

We can see therefore that the scene’s pixels don’t “fit” between the left and right margins as they are stacked up against it. This is the tell-tale characteristic of a scene that has a dynamic range that is larger than that of the camera. In other words, your picture will either have blown-out areas of white or areas of black where no detail is to be seen. It is clear by looking at the picture that this is true. The wall on the right (above the girl’s head) is clearly blown out and the areas on the left, the door and the street, are SO dark, they have become black. Overall this is not a very pretty picture even though the area is a very picturesque one (this is Ouro Preto, a historic gold mining town in Brazil).

This is a perfect scene therefore to consider utilizing the High Dynamic Range technique we mentioned earlier.

Expand the Dynamic Range

When a single shot cannot capture the full dynamic range of a scene (like the one above), you can “cheat” by taking multiple shots, each with a different exposure, then blend them together in a software package. Essentially, this is the HDR process.

You want at least three shots – one with normal exposure, one that is overexposed with enough EV to make sure you have no clipped shadows and one shot that underexposed by enough EV to make sure that you have no blown highlights. Usually, plus and minus 2 stops will do the trick, but make sure you check the histogram to make sure.

Assuming a camera with around 8 stops of dynamic range, taking one shot with 2 stops under and 2 another that is 2 stops over, provides you with 8 +2+2= 12 stops of dynamic range, in other words a 50% INCREASE over your camera’s ability. Don’t be shy when taking these shots. Don’t pick plus and minus a half stop, as that will barely extend the overall DR. Especially when you shoot raw, there is usually enough headroom there to extract one full stop anyway, so make sure you pick at least plus or minus 1 or more.

Can you take more shots than 3? Yes, sure, but more than 5 shots is probably overkill.

Know your Camera

Your camera is your friend when taking multiple shots, as most modern cameras have a feature called Automatic Exposure Bracketing. This feature lets you set an EV value and then all you do it press the shutter and the camera automatically takes multiple shots, each with the EV value you selected. Nikon owners definitely have an advantage here over Canon owners as most Nikon cameras allows you to take 3, 5, 7 or 9 shots automatically up to + or – 5 stops. Canon xxxD and xxD series only do 3 shots up to +/- 2 stops and the 1D series only +/- 3 stops, but will allow 3, 5 or 7 shots.

The HDR Process

So once you have the multiple shots, the process to complete the HDR image is as follows:

Step 1: Take multiple exposures

You can shoot in raw or JPEG

Step 2: Merge them into a 32 bit HDR file using a software package that creates HDR files

This will result in a large file with a dynamic range that cannot really be displayed on your screen. You need to “squish” this 32 bit file back into an 8 or 16 bit file so it can be displayed.

Step 3: Tonemap the HDR file

This process adjusts the file so it can be displayed. This is the process that can either render a surreal looking image. Typically, the result is a JPEG or TIFF image.

Step 4: Further tonal enhancement in your favorite image editor.

This is an optional step, often people just leave it as is after the tonemapping effort. Up to you.

What is tonemapping?

Tonemapping is the process of mapping one set of colors to another. In High Dynamic Range scenarios, it maps the full dynamic range of the HDR file into a displayable range while preserving the image details and color appearance. The algorithms used to do this are called “operators”. There are basically two different kinds, ones that attempt to produce a nice, “normal” looking image (these are called Global Operators) and ones that attempt to reproduce as many of the image details (maximizing contrast) as possible (these are called Local Operators). Either operator is a clever “compromise”. After all, the high dynamic range image contains information we cannot display, so we need to make choices on how to reduce that information in order to see it.

Of these two operators, Local operators (which emphasize contrast) are most likely to create un-realistic looking images with artifacts such as halos, but are the most popular because our eyes are more sensitive to local contrast which is what this kind operator attempts to control best.

While this knowledge this is not critical to making an HDR image, it explains the difference in operators as all HDR software applications offer different operators and you now know what they attempt to do.

What is HDR good for?

This is the $64,000 question, isn’t it? There are in fact many reasons why someone would want to engage in HDR photography, lets look at some of them:

Commercial Purposes

HDR can be a great solution for Architecture, Real Estate and Home Decor photography. Especially high contrast scenes such as a room in a house, with windows looking out to a bright, sunlit back yard.

Artistic Pursuit

Artists can take advantage of the side effect of these un-realistic results of tonemapping and emphasize that, creating stunning works that resemble paintings rather than photos. While these types of images invoke very polarized opinions, I find some of them quite stunning.

Landscape

Just for Fun

Who said there had to be a reason to engage in HDR photography. Let your imagination run wild and think of ways to express your creativity. Check out this video.

Setting Up The Camera For HDR

So now that we know the background of HDR and why we’d want to do it, how do we go about it? Here is the camera settings and setup you need to be aware of:

1. Use a Tripod and Cable Release

This probably is self explanatory, as you are taking multiple shots of the same scene and then blending them, you will want as minimum movement between shots as possible.

2. Put you camera into Aperture Priority

This ensures that the exposure differences between the shots is achieved through different shutterspeed settings. If you were to change the aperture between shots, you may get different depths of field and that will not look good.

3. Auto Exposure Bracketing

Set this to at least +/-1 better is +/- 2 and take at least 3 shots.

4. Continuous Shooting Mode

While this is not strictly necessary, it makes the process easier as the camera will sequence through al the bracketed shots with a single trigger.

5. Manual Focus

Also not strictly necessary, providing you know you camera and how it will focus. If you are not sure, best to avoid the camera refocusing between shots. Therefore, focus the camera, turn AF off, then shoot your sequence, insuring all shots are focused the same.

6. Flash Off

This would defeat the purpose of taking an HDR! After all, your doing so to capture the dark and light segments. If you use the flash to light he dark areas, why do HDR?

7. Multi Segment Metering

Again not strictly necessary providing you know how your camera meters. If in doubt, use multi segment which will take an average of the entire scene. If you set it to spot or center weighted, you run the risk of metering on an area that is too dark or too light and therefore biasing the overall result of the multiple exposed shots either towards the lighter or darker areas.

8. Shoot raw

Not strictly necessary, but adds extra flexibility in things like White balance and exposure.

9. Select the lowest ISO setting

This may not be obvious, but due the the tonemapping process, the details get enhanced, including any noise. Therefore, try and get image with as little noise you can.

The histogram is your friend

Now you are all set to give this a go. While the + or – 2 stops is a good setting, at the end of the day, the histogram is the ultimate tool to ensure you have captured enough dynamic range. Use the histogram, especially when you are just starting out in HDR. It really is you friend. This is what you want to see:

HDR Applications

Here are links to some of the most popular HDR applications out there. They all have trial versions for you to down load and try before you buy.

Dynamic Photo HDR

This is my favorite application. It is fast, has an extremely elegant user interface and offers lots and lots of features and control over the whole process. It offers the largest number of tonemapping operators of all the HDR applications, eight at last count. It is my personal recommendation and I will be posting some tutorials on this excellent tool soon. (disclaimer: I have no affiliation with this company, nor do I get any commission in recommending them, I just like their software).

Photomatix

Photomatix is probably the most widely used application out there. It is very good and there are lots of tutorials to be found on the internet. It offers two tonemapping operators, one Global and one Local.